Sensory perception is an active process. Active sensing means the environment is sampled and then attention drawn to the important target. Just as rapid eye movement precedes fixation on a certain spot, sniffing the area enables selection of an air-borne plume or surface-borne trail.

ACTIVE SNIFFING blazes a trail in mouse behaviour

For a rodent, sniffing, otherwise known as voluntary inhalation, happens at up to four times the baseline respiration rate. The animal decides when and where to sample space, thereby updating an internal representation of its environment that ultimately guides its behaviour. However, unlike vision and audition for which delivering stimuli can be done easily, e.g. with monitors or loudspeakers, delivering odours in a precise way has remained challenging.

With EU funding, the ACTIVE SNIFFING project investigated active sensing in the context of olfactory trail tracking in mice and its neuronal basis. To do so, “We devised a high-throughput experimental system that delivers odour trails to a belt in real-time by using inkjet-printing technology,” outlines project coordinator, Prof. Matthias Bethge.

Results showed that after a short habituation phase, mice will track the printed odour trails, and do so over long period of time if a reward is delivered to the trail at random intervals. Prof. Bethge adds, “Ongoing efforts in the Murthy laboratory at Harvard University remain dedicated to analysis of mouse behaviour during trail tracking, leading to characterisation of their trail tracking behaviour.”

Together with collaborators, the ACTIVE SNIFFING researchers also established deep learning methods to accurately reconstruct the movements of various body parts and especially the snout during trail tracking from videography. They then could accurately measure the movement of the snout without interfering with the natural movement of the mice. Results have been published as a preprint at Cornell University Library.

Tracking the odour trail

The setup is simple but very effective. Paper, which can be up to a metre broad is pulled over a table by a feeding spool and runs onto a collector spool, which is driven by a powerful electric motor. Using paper ensures that the substrate for the odour trails are clean. Inkjet printers with custom built controllers are used to deliver odour trails on the paper.

Mice can freely run on the paper within a transparent, suspended behavioural box. This box is enclosed in a big chamber that shields sounds and light. Most experiments are carried out under infrared light, which mice cannot see. All this assures that they are not using visual and acoustic cues.

Not just a sense of smell

Sense of smell enhances many every day experiences and can serve as a warning system alerting us of danger (spoiled food, fire, or gas leak) and olfactory deficits may be a sign of other health problems. “First and foremost, we believe that this setup and the trail tracking behaviour that we can study with it, allows us to understand olfaction in unprecedented ways.”

A basic understanding of how sensory information is processed and represented in the brain is an important scientific goal of neuroscience. As Prof. Murthy explains, “Such an understanding is critical for attempts to uncover sensory disorders in neurological or mental illnesses including Alzheimer’s disease, Parkinson’s disease and autism.”

Figuring out how macrosmatic animals with highly developed sense of smell like mice and dogs track odour trails in algorithmic terms could be of interest for bionic applications. Revealing the parameters of the highly efficient trail-tracking capabilities could be implemented in odour-trail-tracking-robots.

The team are still analysing the data and investigating which strategies mice use for tracking trails in various conditions. They have also collected neural recordings during the tracking behaviour and attempt to understand the neural code during this behaviour.

Already published, papers include two in the high-profile journal Neuron and another five in other peer-reviewed journals.